As a next generation mobile communication system, a LTE (Long Term Evolution) system, which is a mobile communication system enabling data transmissions at 100 Mbps, has been intensively researched and developed.
Such a LTE system may be initially operated with a smaller number of carriers, as illustrated in FIG. 1. For example, the LTE system is initially operated with Band A. If the frequency band becomes insufficient after a few years, license for new Band B may be obtained, or another band used for a currently operated 3G system may be reassigned to the LTE system, so that the LTE system can be operated with Bands A and B. In addition, it is expected that the LTE system may be operated with Bands C and D in future. The term “band” used herein means a frequency band, such as 800 MHz band or 2 GHz band, where the LTE system is operated. Also, the term “carrier” used herein means a bandwidth for a system operated in a frequency band, and the demand may be that the carrier can correspond to any of 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz in the LTE system.
In this situation, there may initially exist only mobile stations or user equipment (UE) capable of transmission and reception in Band A (hereinafter referred to as “Band A capable UEs”), but mobile stations capable of transmission and reception in Bands A and B (hereinafter referred to as “Band A+B capable UEs”) may be additionally provided after a few years. Even if Band B were newly introduced, the Band A capable UEs would not be able to use Band B for transmission and reception.
In the future, mobile stations capable of transmission and reception in Bands A, B, C and D (hereinafter referred to as “Band A+B+C+D capable UEs”) may be additionally introduced. Thus, the same operator's network may be operated with multiple bands and multiple carriers, and different types of mobile stations with different transmission and reception capabilities may coexist.
If it comes to this situation, load balancing that can handle different transmission and reception capabilities of mobile stations may be required. For example, in a system operated with several bands and/or carriers, if mobile stations are concentrated in a certain carrier, the carrier may be intensively used for transmission and reception despite other bands and/or carriers being not fully used, resulting in degraded communication quality.
For the load balancing, there may be two types of load balancing schemes, traffic load balancing and camp load balancing. In the traffic load balancing, active users, that is, presently communicating users, may be uniformly distributed. In the camp load balancing, idle users, that is, users waiting for call, may be uniformly distributed.
For example, an article written by J. Laiho, A. Wacker and T. Novosad “Radio Network Planning and Optimisation for UMTS” (John Wiley & Sons, Chichester, 2002, p. 229-231) describes UMTS-GSM load balancing in conjunction with UMTS cell design.
Also, an article “Load sharing using cell reselection” (T-Mobile, R2-060934, TSG-RAN WG2 #52, Athens, Mar. 27-31, 2006) describes LTE-UMTS load balancing particularly for cases of where multiple operators share a network.
However, the load balancing has not been discussed from the viewpoint of separation between the traffic load balancing and the camp load balancing.